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Keck-funded group proposes new topological superconductor

November 21st, 2017 ›

The metal-quantum paramagnet heterostructure
proposed by a research group led by Eun-Ah Kim. The metal provides the charge
carriers; the QPM provides a pairing interaction via spin fluctuations.

The Keck Foundation announced in
early July that it had awarded $1 million to a Cornell cross-campus
collaboration of professors in engineering and physics aimed at turning theory
into reality - namely, creating a specific topological superconducting material
that could help pave the way to quantum computing.

The idea that sparked the group's
winning proposal came out of the group led by Eun-Ah
Kim, associate professor of physics, and is now the first published
research from a member of that five-member group.

Other contributors included Craig Fennie, associate professor in
applied and engineering physics, former postdoc Choong Kim of the Fennie Group,
and Michael Lawler, adjunct associate professor of physics and associate
professor at Binghamton University.

Kim and Fennie are part of the
collaboration that won the Keck Foundation grant, as are engineering professor Darrell Schlom, who heads the group, and
physics professors J.C. Seamus Davis and Kyle Shen.

Quantum spin ices are magnetic
materials in which fluctuations in their basic energy states prevent long-range
magnetic ordering. Such "frustration," as this state of flux is known, has long
been thought to be good for superconducting, but the ability to control these
fluctuations has been the challenge.

"To be predictive about
superconductivity has always been hard; it's an interaction-driven phenomenon,"
Kim said.

What's needed in a superconductor
is a mobile charge carrier to deliver the current and a facilitating
interaction that cause "Cooper" pairing of electrons, that normally repel each
other. The group's novel approach: Bring the carriers and the pairing
interaction from separate materials to control them individually instead of
"letting it all happen in a soup of a bulk material," Kim said.

To materialize this particular
superconducting phase, the group proposes a film of a metal compound
(Y2Sn2-xSbxO7) to be grown on the surface of a quantum spin ice material
(Pr2Zr2O7). These are both well-understood compounds; each contributes one of
the two essential ingredients for superconducting.

"We wanted to exercise and test our
control by being predictive in terms of what kind of superconductor we were
going to get," Kim said.

Due to the understood nature of the
constituent materials, the group predicted that the heterostructure would have
the ability to facilitate the topological pairing of electrons whose spins
point in the same direction. It also predicted odd-parity superconducting at
around 1 degree Kelvin, comparable to strontium ruthenate (1.5K), the only
other current solid-state candidate for topological superconducting.

Future work will examine other
substrate-metal combinations, Kim said, although she notes in the paper that
likely the biggest challenge will be in controlling disorder, which is
unavoidable with chemical doping (modulation) of the metallic layer.

In addition to the Keck Foundation,
support for this research came from the U.S. Department of Energy, Office of
Basic Energy Sciences; and the National Science Foundation.